A communication channel between modems may include a data communication network such as a T1 or other digital communication link in which data is transmitted in frames. The frames and time slots associated with the data must be synchronized in order to ensure that the received data is the data transmitted by the corresponding transmitting device, not data from another source or out of sequence data from the correct data source.
Errors in received data can be classified into three categories. The first is an occasional bit error such as due to white Gaussian noise of a moderate amplitude relative to the data. The second is a burst bit error where several consecutive bits have errors due to an impulsive or transient type of noise. Burst bit errors can occur due to relatively short undesired amplitude or phase excursions. The third category of errors consists of received data which is totally missing or received data which is totally different from the anticipated transmitted data such as might occur in a time division multiplexed (TDM) digital signalling network. Such errors may result from a frame synchronization error or "slip" in a TDM network.
A loss of frame timing due to burst bit errors usually results in a need to reinitialize the modems. The reinitialization command can be generated by a time-out timer in a data communication front end device (FED) connected to the receiving modem. It would be desirable for the modem itself to detect and handle such a problem rather than responding to a later requested reinitialization command by the FED. This would minimize the amount of erroneous data that has to be retransmitted and re-establish proper data communications with minimum delay.
A frame slip cannot be detected by normal processes and will usually require reinitialization of a modem especially where multiple users are sharing the same modem. Thus, it would be desirable for the modem to be able to sense a loss of frame synchronization in order to initiate reinitialization as soon as possible.
It is an object of the present invention to provide an improved modem and method suited for use with modems for detecting frame synchronization errors associated with the received data.